“The average of these specific gravities is 7.562.
“The average before experiment was 7.64. Hence the average loss in specific gravity has been 1.02 per cent.
“The small triangular piece No. 1a, specific gravity 7.552 (already subjected to fifty heatings when forming part of the solid plate), was next heated and cooled fifty times more. The specific gravity at the end of the one hundred total coolings was 7.52, being .43 per cent. lower than after fifty heatings in plate, and 1.57 per cent. lower than 7.64, the original mean specific gravity of the plate.
“The same piece, 1a, was then heated twenty-five times more, making 125 in all. On taking the specific gravity it was found to be 7.526, or practically the same as after 100 total heatings and coolings.
“It thus appears that there is an undoubted decrease in specific gravity on repeated heating and cooling as described up to one hundred coolings, the specific gravity decreasing as much as 1.57 per cent.; that this percentage appears to be less when the pieces of iron operated upon are very small; that while there is a decrease of specific gravity there is also a decrease of total volume.
“From the above it was evident that the volume was affected by several causes:—
“1. By the permanent contraction of the outer skin, either the volume would be lessened, or relief by bulging out the sides must occur.
“2. By the decrease of specific gravity an increase of volume must occur, which could also find relief in bulging.
“3. A diminution of the whole mass must occur through scaling of the surface.
“Having determined the change in specific gravity by Experiment 6, we only now want to determine the loss of volume due to surface scaling, and we can then infer the actual contraction of the outer skin.